1. Field of the Invention
The present invention relates to winding cores and, more particularly, to winding sheets of paper, film, and the like into large rolls and a method of winding such sheets onto a core.
2. Description of Related Art
Web materials such as polymer film, paper, nonwoven or woven textile, metal foil, sheet metal, and others, are used to manufacture a variety of products. The web materials are generally provided in the form of large rolls formed by winding the web material about a winding core. The core is generally paperboard, though it may be reinforced with a plastic outer shell or the like. The paperboard may be formed of high strength, high density paperboard plies. A roll of paper or the like wound onto the core typically has a weight above two tons and often exceeding five tons. Typical core sizes are an internal diameter of 3 in. (76.2 mm.) to 6 in. (152.0 mm.) or 150.4 mm. in Europe, and a length of about 100 to 140 in. To begin the winding process, a tail end of a web is attached to the winding core and the core is rotated about its axis to wind the web into a roll. The rolls are subsequently unwound during a printing or similar process.
Web converters such as printers or the like continually strive to increase productivity of converting processes by increasing the total amount of web throughput per unit time. To this end, there has been a continual push toward wider webs and higher web speeds, which lead to longer winding cores that must rotate at higher rotational speeds and must support heavier rolls of the wider web material. For instance, rotogravure printers are currently developing 4.32 m. wide printing presses for high-speed printing. Paper supply rolls for such presses would weigh in excess of 7 tons. Applications such as this place extreme demands on the stability of current winding cores. A potential solution to the problem is to increase core stiffness by increasing core diameter, but this would be undesirable if it meant that the cores would not be compatible with existing winding and unwinding machinery, as would be the case if the inside diameter of the core were increased.
During a winding or unwinding operation, a core is typically mounted on a rotating expandable chuck that is inserted into each end of the core and expanded to grip the inside of the core so that the core tends not to slip relative to the chuck as torque is applied therebetween. Typically, the rotation of the core is achieved by means of a drive coupled to one or both of the chucks, and the core is rotated to achieve web speeds of, for example, 15 to 16 m/s. The rolls of material are often subjected to substantial circumferential acceleration and deceleration by the winding machines. This, in turn, subjects the engaged ends of the paperboard roll to substantial torque forces. This often leads to some slippage of the chuck on the inside of the core. In an extreme situation, the slippage can lead to “chew-out” wherein the core is essentially destroyed by the chuck.
Aside from problems such as chew-out, the failure of the chuck to firmly grip the core can lead to other undesirable effects. In particular, it has been discovered that it can lead to a reduction in the “chuck factor” of the core, which is defined as the resonant frequency of the core when chucked, divided by the resonant frequency of the core when free. It is desirable for the chuck factor to be as high as possible without risking excessive vibration. The natural frequency of vibration of a core corresponds to that core's resonant frequency and may be calculated using the formula:
  F  =                    22.4        ×                  C          r                            2        ⁢        π              ×                  (                              E            ×            I                                m            ×                          L              3                                      )                    1        2            where F is the natural frequency of the core while chucked, Cr is the relative chuck factor, E is the modulus of elasticity of the core along its length, I is the moment of inertia, m is the mass of the core, and L is the length of the core.
Efficient winding requires that the natural frequency of the chucked core be higher than the core rotational speed during winding and unwinding, where the natural frequency depends upon the above factors and the way it is supported by the chucks. A safety factor of 15 to 20% is typically taken into account, as there should be assurance that the maximum rotational frequency of the core while chucked will remain less than the natural frequency of the core. Current winding cores generally produce chuck factors of about 0.70 to 0.80, which limits the percent safety factor and winding speed of the core without risking excessive vibration.
Accordingly, a need exists for an improved core that provides better grip to prevent the chuck from slipping and possibly damaging the core during winding and unwinding. In addition, a need exists for a core that provides for an improved chuck factor.